08-Jul-2019, 11:02
@thumb5
Your maths are correct in that if your normal listening level requires 5 W and you want to produce peaks that are 20 dB louder then you need to be able to deliver at least 500 W.
The premises/assumptions in your reasoning that I disagree with:
1- that most people needs 5 W for "normal listening level", even with 85 dB sensitive speakers. My normal listening level is around the 80 dB mark. My listening distance is a bit over 2 metres and an 85 dB sensitive speaker would produce roughly 80 dB levels for 1 W output at my listening distance but I do listen in stereo and that means that 2 speakers each producing around 80 dB for 1 W output will deliver an 83 dB listening level at my listening position. We get a 3 dB gain in efficiency from having 2 speakers so I actually need a little less than 1 W to produce an 80 dB listening level at my listening position. OK, my preferred listening levels are on the low side for many people but I do think that 5 W is a little on the high side even for low sensitivity speakers given that we're using 2 speakers to produce that listening level.
2- In order to produce a 20 dB higher peak than our normal listening level we do need to multiply the output required to produce the normal listening level by a factor of 100. Whether or not the music a given person listens to is going to have a crest factor of 20 dB is arguable. It's definitely possible with classical music and other well mastered acoustic music but I think the crest factor for a lot of pop/rock music with limited dynamic range is likely to be significantly less than 20 dB. Depending on your taste in music you may not need to accommodate a crest factor of 20 dB.
3- Even accepting your 5 W starting point for normal listening level and the 500 W needed to faithfully deliver a crest factor of 20 dB, that doesn't mean you need an amp capable of delivering a continuous 500 W output and amplifier power ratings are for continuous output. Amplifiers are capable of delivering more than their continuous output rating for transients without clipping. I've got no idea what output my 140 is capable of delivering on a transient but my normal listening level would require less than 1 W per channel, a 20 dB crest factor would require it to deliver transient peaks of 100 W a channel, and the 140 is rated at 140 W continuous output per channel into 6 Ohms and my speakers are 4 Ohms. That means it's more than capable of delivering a 20 dB crest factor without clipping given my normal listening levels, and it could probably meet that 20 dB crest factor requirement for someone who needed twice as much power per channel for their normal listening level.
Given that we use 2 speakers, that most speakers aren't low sensitivity, and that we don't need an amp capable of continuous output at the level required for instantaneous peaks, I wonder just how real world your 500 W example is. My feeling is that the 140 Pro is sufficient for quite a few people and that the 220 and 250 are adequate for most people who need more than the 140. Some people will definitely require a dual amp setup and there are certainly speakers which place more demanding loads on an amp and benefit from larger amps than these figures suggest plus I think there are some sound quality gains from the dual amps that some people find attractive. I think real world maths suggest that we don't need an amp capable of 400-500 W/ch but there are certainly some people who will need that given their choice of speaker, listening level, and room size and there are others who go for the dual amp setups for reasons unrelated to power requirements.
Your maths are correct in that if your normal listening level requires 5 W and you want to produce peaks that are 20 dB louder then you need to be able to deliver at least 500 W.
The premises/assumptions in your reasoning that I disagree with:
1- that most people needs 5 W for "normal listening level", even with 85 dB sensitive speakers. My normal listening level is around the 80 dB mark. My listening distance is a bit over 2 metres and an 85 dB sensitive speaker would produce roughly 80 dB levels for 1 W output at my listening distance but I do listen in stereo and that means that 2 speakers each producing around 80 dB for 1 W output will deliver an 83 dB listening level at my listening position. We get a 3 dB gain in efficiency from having 2 speakers so I actually need a little less than 1 W to produce an 80 dB listening level at my listening position. OK, my preferred listening levels are on the low side for many people but I do think that 5 W is a little on the high side even for low sensitivity speakers given that we're using 2 speakers to produce that listening level.
2- In order to produce a 20 dB higher peak than our normal listening level we do need to multiply the output required to produce the normal listening level by a factor of 100. Whether or not the music a given person listens to is going to have a crest factor of 20 dB is arguable. It's definitely possible with classical music and other well mastered acoustic music but I think the crest factor for a lot of pop/rock music with limited dynamic range is likely to be significantly less than 20 dB. Depending on your taste in music you may not need to accommodate a crest factor of 20 dB.
3- Even accepting your 5 W starting point for normal listening level and the 500 W needed to faithfully deliver a crest factor of 20 dB, that doesn't mean you need an amp capable of delivering a continuous 500 W output and amplifier power ratings are for continuous output. Amplifiers are capable of delivering more than their continuous output rating for transients without clipping. I've got no idea what output my 140 is capable of delivering on a transient but my normal listening level would require less than 1 W per channel, a 20 dB crest factor would require it to deliver transient peaks of 100 W a channel, and the 140 is rated at 140 W continuous output per channel into 6 Ohms and my speakers are 4 Ohms. That means it's more than capable of delivering a 20 dB crest factor without clipping given my normal listening levels, and it could probably meet that 20 dB crest factor requirement for someone who needed twice as much power per channel for their normal listening level.
Given that we use 2 speakers, that most speakers aren't low sensitivity, and that we don't need an amp capable of continuous output at the level required for instantaneous peaks, I wonder just how real world your 500 W example is. My feeling is that the 140 Pro is sufficient for quite a few people and that the 220 and 250 are adequate for most people who need more than the 140. Some people will definitely require a dual amp setup and there are certainly speakers which place more demanding loads on an amp and benefit from larger amps than these figures suggest plus I think there are some sound quality gains from the dual amps that some people find attractive. I think real world maths suggest that we don't need an amp capable of 400-500 W/ch but there are certainly some people who will need that given their choice of speaker, listening level, and room size and there are others who go for the dual amp setups for reasons unrelated to power requirements.
Roon Nucleus+, Devilalet Expert 140 Pro CI, Focal Sopra 2, PS Audio P12, Keces P8 LPS, Uptone Audio EtherREGEN with optical fibre link to my router, Shunyata Alpha NR and Sigma NR power cables, Shunyata Sigma ethernet cables, Shunyata Alpha V2 speaker cables, Grand Prix Audio Monaco rack, RealTRAPS acoustic treatment.
Brisbane, Qld, Australia
